Plasma Spray Method

ABSTRACT

The invention relates to a plasma spray method which can serve as a starting point for a manufacture of metal nanopowder, nitride nanopowder or carbide nanopowder or metal films, nitride films or carbide films. To achieve an inexpensive manufacture of the nanopowder or of the film, in the plasma spray in accordance with the invention a starting material (P) which contains a metal or silicon oxide is introduced into a plasma jet ( 113 ) at a process pressure of at most  1000  Pa, in particular at most  400  Pa. The starting material (P) contains a metal or silicon oxide which vaporizes in the plasma jet ( 113 ) and is reduced in so doing. After the reduction, the metal or silicon which formed the metal or silicon oxide in the starting material is thus present in pure form or in almost pure form. The metal or silicon can be deposited in the form of nanopowder or of a film ( 124 ). Nitride nanoparticles or films or carbide nanoparticles or films can be generated inexpensively by addition of a reactant (R) containing nitrogen or carbon.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 of European Patent Application No. 12156660.8 filed on Feb. 23, 2012, the disclosure of which is expressly incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A COMPACT DISK APPENDIX

Not applicable.

BACKGROUND OF THE INVENTION

It is known that nitride nanopowder or carbide nanopowder can be manufactured using suitable liquid or gaseous starting materials, so-called precursors. In this connection, a nanopowder should be understood as a powder having a grain size of approximately 1 nm to 1 μm. Suitable precursors, for example titanium tetrachloride or tetrakis (dimethylamino) titanium, are very expensive and usually very toxic or dangerous. The precursors are vaporized for manufacturing the nanopowder and form nanopowder in a reactive chemical gas phase deposition process (a so-called CVD process). It is also possible to manufacture a nitride coating or a carbide coating on a substrate using a comparable process.

In addition, so-called plasma spray gas phase deposition processes (so-called PS-PVD processes) are known by means of which films can be generated on a substrate from a starting material in powder form. For this purpose, the starting material is introduced into a plasma and thus converted into the gas phase and is deposited from the gas phase onto the substrate as a thin film. Thermal barrier coatings are produced in this manner, for example. In this respect, yttria stabilized zirconia (abbreviated YSZ) is, for example used as a non-metallic, inorganic starting material.

SUMMARY

Against this background, it is the object of the invention to propose a plasma spray method which can serve as a starting point for an inexpensive manufacture of metal nanopowder, nitride nanopowder or carbide nanopowder or of metal films, nitride films or carbide films. This object is satisfied in accordance with the invention by a plasma spray method having the features of claim 1.

In the plasma spray method in accordance with the invention, a starting material is introduced into a plasma jet generated by a plasma generator at a process pressure of at most 1000 Pa, in particular at most 400 Pa. The starting material contains a metal or silicon oxide which vaporizes in the plasma jet and is reduced in so doing. After the reduction, the metal or silicon which had formed the metal or silicon oxide in the starting material is thus present in pure form or in almost pure form.

The term “process pressure” should be understood as that pressure at which the process runs, that is at which the plasma jet is formed. Since the required process pressure is much smaller than the atmospheric pressure, the whole process takes place in a closed process chamber in which the process pressure can be set.

The starting material in this respect is composed of in particular 95% to 100% metal oxide, particularly preferably 100% of metal oxide, of a single metal or of different metals, in particular of zirconium oxide (zirconia), hafnium oxide or titanium oxide. In addition to the metal oxide, the starting material can, for example, be composed of other oxides, for example silicon oxide.

In the method in accordance with the invention, the starting material containing metal or silicon oxide is injected and thus introduced into a plasma jet, for example an argon-helium plasma, generated by a plasma torch known per se. The starting material is in this respect in particular introduced into the plasma jet as a powder. It can, for example, be introduced as a loose powder by means of a carrier gas. The carrier gas is, for example, a noble gas, a noble gas mixture or an inert gas. Examples for carrier gases are argon or a helium-argon mixture. It is, however, also possible that the starting material is introduced into the plasma jet in a suspension, that is as a dispersed powder in a liquid, for example in ethanol,

The plasma gas thereby arising expands in a nozzle of the plasma torch due to the high temperature of the plasma from 10,000 to 20,000 Kelvin and accelerates to supersonic speed. Due to the named low process pressure, an expansion of the plasma gas takes place into a process chamber at low pressure, with a long, large-area plasma jet with expansion zones and compression zones arising. The plasma jet in particular has a length between 1 and 2.5 m. The metal or silicon oxide contained in the starting material vaporizes in the plasma jet due to the high temperature and to the low pressure. In this respect, an oxygen loss, that is a reduction of the metal or silicon oxide, takes place due to the low partial pressure of the oxygen so that the metal or silicon is present in pure form or in almost pure form in the plasma flow after the reduction.

The starting material is in particular supplied and is thus introduced into the plasma at a comparatively low feed rate. The supply rate in particular lies in a range between 0.1 and 5 g/min. It can thus be achieved that the total metal or silicon oxide or almost the total metal oxide introduced is completely reduced.

The total flow rate of the process gate in particular amounts to between 50 and 200 SLPM (standard liters per minute) and particularly preferably to 90 to 120 SLPM.

In an embodiment of the invention, an additional reactant is introduced into the plasma so that a reaction can take place between the reduced metal or silicion oxide and the reactant to form a reaction product. The reactant in particular contains nitrogen and/or carbon so that the pure metal or silicon arising on the reduction can react to form a metal or silicon nitride and/or a metal or silicon carbide. It is thus advantageously possible to manufacture metal or silicon nitride and/or metal or silicon carbide from a very inexpensive and non-dangerous starting material in the form of metal or silicon oxide powder.

Depending on the partial pressure of the elements in the reaction to the reaction product, MO_(x)N_(y) or MO_(x)C_(y) or MN_(y) or MC_(y), arise, where M stands for the metal forming the metal oxide, for example zirconium or titanium. The named partial pressures can be influenced by means of process parameters such as the process pressure, type of process gas and the current for generating the plasma, the flow rate of the process gas or the supply rate of the starting material. A large reduction is in particular achieved by a high current as well as by a low powder conveying rate.

The reactant containing nitrogen can, for example, be pure gaseous nitrogen or air. The reactant containing carbon can, for example, be gaseous as carbon dioxide or methane or can be in solid form as starch or as a polymer.

The pure metal, the silicon or the reaction product arising on the reduction can be deposited from the plasma jet. It can be deposited, for example, in the form of metal or silicon nanopowder. The deposition in particular takes place at a comparatively small spacing from a discharge nozzle for the plasma jet. The spacing in particular amounts to between 100 and 400 mm. The named range is characterized in that, on the one hand, the reduction of the metal or silicon oxide is fully or at least almost fully completed and, on the other hand, the metal or silicon particles have not yet entered into any other bonds. The named spacing is in particular advantageous when metal or silicon nanopowder is to be manufactured.

The plasma spray method in accordance with the invention allows an inexpensive manufacture of metal or silicon nanopowder, nitride nanopowder or carbide nanopowder. In addition, with corresponding process conditions, nanopowders can thus be manufactured from non-meltable nitride compounds or carbide compounds such as silicon nitride (Si₃N₄).

A method is thus proposed for manufacturing a metal or silicon powder, a metal or silicon nitride powder or a metal or silicon carbide powder in which a starting material in the form of a metal or silicon oxide is used which is introduced at a process pressure of at most 1000 Pa into a plasma flow which is generated by a plasma generator and in which the starting material is vaporized and in so doing reduced and arising metal particles, metal nitride particles or metal carbide particles are deposited from the process jet as powder.

The deposition as a nanopowder in particular takes place when the plasma jet can form without impacting a barrier, for example in the form of a substrate. To promote the deposition as a nanopowder, the plasma jet can also be directly cooled in a defined region, for example by means of a gas flow; the metal or silicon can therefore be quenched so-to-say and the formation of metal or silicon nanopowder can thus be promoted. The gas flow is, for example, a gas flow from a noble gas (e.g. argon), from a noble gas mixture (e.g. argon-helium mixture) or from an inert gas and is oriented transversely to the plasma jet, for example. An electrostatic filter can also be used for promoting the deposition.

The grain size of the arising nanopowder can in particular be influenced by the type of the plasma gas which has an influence on the plasma temperature and the plasma speed and by the process pressure which has an influence on the condensation of the nanopowder. The grain size in particular becomes smaller as the current increases and the powder conveying rate decreases.

The pure metal or silicon or the reaction product created in the reduction can also be deposited from the plasma jet as a film on a substrate. A substrate should be understood in this connection as a workpiece to be coated, for example a turbine blade. For this purpose, the plasma jet is directed to the substrate so that a film of the pure metal or silicon or of the reaction product is formed on the substrate. The plasma spray method in accordance with the invention thus makes possible an inexpensive metal or silicon coating, nitride coating or carbide coating of a substrate. In addition, on the setting of corresponding process parameters using the method in accordance with the invention coatings of non-meltable nitride compounds or carbide compounds such as silicon nitride (Si₃N₄) can be manufactured. This is not possible or is only possible with limitations with known spray methods for coating.

A method is thus proposed for manufacturing a film on a substrate using a plasma spray method in which a starting material in the form of a metal oxide is used which is introduced at a process pressure of at most 1000 Pa into a plasma flow which is generated by a plasma generator and in which the starting material is vaporized and in so doing reduced and arising metal particles, metal nitride particles or metal carbide particles are deposited as a film on s substrate.

The films produced in particular have a thickness between 50 nm and 500 μm and can be deposited over a large area in both a dense and a porous form. The porous films in particular have a columnar design. The design and the property of the film as well as the film growth can be influenced via the named process parameters, with in particular a higher powder conveying rate resulting in rather porous films.

Further advantages, features and details of the invention result with reference to the following description from embodiments and with reference to drawings in which elements which are the same or have the same function are provided with identical reference numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown:

FIG. 1 a schematic representation of a plasma spray apparatus for manufacturing nanopowder; and

FIG. 2 a schematic representation of a plasma spray apparatus for producing a film on a substrate.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with FIG. 1, a plasma spray apparatus 11 suitable for carrying out a method in accordance with the invention has a plasma generator 12 known per se and having a plasma torch, not shown in more detail, for producing a plasma. A process jet 13 is generated in a manner known per se from a starting material P, a process gas mixture G and electrical energy E using the plasma generator 12. The feeding of these components E, G and P is symbolized by the arrows 14, 15, 16 in FIG. 1. The generated plasma jet 13 exits the plasma generator through an outlet nozzle 17 and transports the starting material P in the form of the plasma jet 13 in which material particles 18 are dispersed in a plasma. This transport is symbolized by an arrow 19.

The process gas G for the production of the plasma is preferably a mixture of inert gases, in particular a mixture of argon, hydrogen and helium.

The plasma spray apparatus 11 is arranged in a process chamber 20 in which a defined process pressure can be set by means of pumps, not shown. On the carrying out of the method in accordance with the invention, a process pressure of less than 1000 Pa, in particular between 100 and 400 Pa, is set. Due to the named process pressure, a comparatively long plasma jet having a length between 1 and 2.5 m is produced.

Specifically, a plasma spray gas deposition method (PS-PVD) is carried out using the plasma spray apparatus 11 shown in FIG. 1. In this method, the starting material P which is composed of titanium oxide (TiO₂, zirconia (ZrO₂), hafnium oxide (Hf₂O₃) or silica (SiO₂) in powder form is introduced into the argon-helium plasma generated by the plasma generator 12, and thus introduced into the plasma jet 13, by means of a carrier gas, for example in the form of argon.

The plasma gas thereby arising expands in and after the exit from the outlet nozzle 17 of the plasma generator 12 due to the high temperature of the plasma of 10,000 to 20,000 Kelvin and accelerates to supersonic speed. The metal oxide contained in the starting material P vaporizes in the plasma jet 13 due to the high temperature and to the low pressure. In this respect, an oxygen loss, that is a reduction of the metal oxide, takes place due to the low partial pressure of the oxygen so that the metal is present in pure form or in almost pure form in the plasma flow after the reduction.

The starting material P is supplied at a comparatively low supply rate. The supply rate in particular lies in a range between 0.1 and 5 g/min.

The current set for the generation of the plasma in this respect has a current between approx. 1000 and 3000 A, in particular between 2200 and 3000 A.

The starting material P is injected into the plasma as a powder jet with a conveying gas, preferably argon or a helium-argon mixture. The flow rate of the conveying gas preferably amounts to 5 to 40 SLPM (standard liters per minute), in particular to 10 to 25 SLPM.

The process gas for the generation of the plasma is preferably a mixture of inert gases, in particular a mixture of argon Ar, helium He and hydrogen H. In practice, a total gas flow between 50 and 200 SLPM, in particular 90 to 120 SLPM has proven itself. Of this in particular approximately ⅓ is argon and ⅔ helium. In addition, a portion of up to 10 SLPM hydrogen is conceivable.

To promote a deposition of the created metal particles in the form of a nanopowder, the plasma jet 13, and thus also the metal particles 18 contained in the plasma jet 13, are directly cooled by a gas flow 21 oriented transversely to the plasma flow 13 and the metal is thus quenched so-to-say. A condensation of the gaseous metal particles 18 is triggered and nanopowder 22 is formed which collects in a collection apparatus 23.

The gas flow 21 in this respect has a spacing D1 from the exit nozzle 17 between 100 and 400 mm, in particular 150 mm.

A plasma spray apparatus 111 for generating a film 124 on a substrate 125 is shown in FIG. 2. The design of the plasma spray apparatus 111 corresponds in large parts to the plasma spray apparatus 11 of FIG. 1 so that mainly the differences of the two plasma spray apparatus will be looked at.

A reactant R, whose infeed is symbolized by an arrow 126, is supplied to a plasma generator 112 of the plasma spray apparatus 111 in addition to the starting material P which is in turn titanium oxide (TiO₂) or zirconia (ZrO₂) in powder form. The reactant in particular contains nitrogen or carbon so that the pure metal arising on the reduction of the metal oxide can react to form a metal nitride and/or a metal carbide. The reactant containing nitrogen can, for example, be pure gaseous nitrogen. The reactant containing carbon can, for example, be gaseous as carbon dioxide or methane or can be in solid form as starch or as a polymer. When the reactant is supplied in solid form, this is also done using a transport gas. In this respect, it can be the same transport gas as for the starting material P or a transport gas differing therefrom.

After the above-described reduction of the metal oxide of the starting material P, a reaction of the pure metal with the reactant R to form a reaction product takes place in the plasma flow 113. If one looks at the total method, the oxygen bound in the metal oxide is replaced with nitrogen or carbon of the reactant R to gain the reaction product.

Depending on the partial pressure of the elements on the reaction to the reaction product, MO_(x)N_(y) or MO_(x)C_(y) or MN_(y) or MC_(I), arise, where M stands for the metal forming the metal oxide, that is zirconium or titanium. The named partial pressures can be influenced by means of process parameters such as the process pressure, type of process gas and the current for generating the plasma, the flow rate of the process gas or the supply rate of the starting material.

The substrate 125 is arranged at a spacing D2 from an exit nozzle 117 of the plasma generator 112. The spacing D2 can be selected as larger than the spacing D1 in FIG. 1; it in particular amounts to between 500 and 2000 mm. The named reaction product is deposited at the substrate 125 as a film 124. The film 124 can have a thickness between 50 nm and 500 μm and can be made both dense and porous. The design and the property of the film as well as the film growth can be influenced via the named process parameters.

It is naturally also possible that, in the generation of nanopowder in accordance with FIG. 1, a reactant is supplied and thus a nitride nanopowder or carbide nanopowder is generated. It is equally possible to dispense with the supply of a reactant in the generation of a film on a substrate in accordance with FIG. 2 and thus to generate a film of pure metal. To achieve satisfactory results, specific adaptations of the process parameters will be necessary under certain circumstances.

It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to an exemplary embodiment, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and sprit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. 

1. A plasma spray method, in which a starting material is introduced into a plasma jet generated by a plasma generator at a process pressure in a process chamber of at most 1000 Pa, wherein the starting material contains a metal or silicon oxide which is vaporized in the plasma jet and is therefore reduced in so doing.
 2. A plasma spray method in accordance with claim 1, characterized in that the starting material is composed only of metal oxide of a single metal.
 3. A plasma spray method in accordance with claim 2, characterized in that the starting material is introduced into the plasma jet as a powder.
 4. A plasma spray method in accordance with claim 1, characterized in that the metal oxide is zirconia, hafnium oxide or titanium oxide.
 5. A plasma spray method in accordance with claim 1, characterized in that the process pressure amounts to at most 400 Pa.
 6. A plasma spray method in accordance with claim 1, characterized in that a supply rate of the starting material lies in a range between 0.5 and up to 5 g/min.
 7. A plasma spray method in accordance with claim 1, characterized in that metal or silicon arising in the reduction of the metal or silicon oxide is deposited from the plasma jet.
 8. A plasma spray method in accordance with claim 7, characterized in that the metal or silicon is deposited in form of particles at a spacing between 100 and 400 mm remote from an exit nozzle for the plasma jet.
 9. A plasma spray method in accordance with claim 1, characterized in that an additional reactant is introduced into the plasma jet and a reaction can thus take place between the reduced metal oxide and the reactant to form a reaction product.
 10. A plasma spray method in accordance with claim 9, characterized in that the reactant contains nitrogen and/or carbon.
 11. A plasma spray method in accordance with claim 9, characterized in that particles of the reaction product are deposited from the plasma jet.
 12. A plasma spray method in accordance with claim 11, characterized in that the particles of the reaction product are deposited at a spacing between 500 and 2000 mm remote from an exit nozzle for the plasma jet.
 13. A plasma spray method in accordance with claim 7, characterized in that the metal particles or the particles of the reaction product are deposited in the form of a powder.
 14. A plasma spray method in accordance with claim 7, characterized in that the metal particles or the particles of the reaction product are deposited in the form of a film on a substrate. 